larger tiles are not always a no no in a smaller space- sometimes it does make the space look larger and I do this often. However, your bathroom is very small and once you add in the tub/shower, toilet and vanity very little floor space will remain. Therefore I suggest you stick with the smaller floor tiles. You can also use the planks if they are not too large, but I would stay away from the large square or rectangle tiles in this instance.

Too large tiles make it seem smaller. And will cost you more in difficulty of setting labor. Be sure you understand proper waterproofing. Not a lot of “tile guys” do. You need an actual licensed and insured professional who will have the waterproofing conversation with you.

Optical illusion , slightly larger tiles will make it look "like a larger area" smaller ones make it look too busy. Also less grout which is always a good idea even thought grout has improved massively in recent years.

Visually it is the grid lines that the grout joints create. Larger will give you a cleaner look although B&W mosaics are a classic depends on the look you are going for.

I always suggest grout matches the tiles as well as possible - to avoid the grid line - its my pet hate !!

Never say always. There are certainly times when a contrasting grout is required to achieve specific look.